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Quaternary
The Quaternary Period is the geologic time period from the end of the Pliocene Epoch roughly 1.806 million years ago to the present. The Quaternary includes 2 geologic subdivisions -- the Pleistocene and the Holocene Epochs. In a recent revision of the international classification of geological time periods, the Quaternary was subsumed into the Neogene. The move has met with some resistance from geologists. Overview The term Quaternary ("fourth") was proposed by Jules Desnoyers in 1829 to address sediments of France's Seine Basin that seemed clearly to be younger than Tertiary Period rocks. The Quaternary Period follows the Tertiary Period and extends to the present. The Quaternary roughly covers the time span of recent glaciations, including the last glacial retreat. An occasional alternative usage places the start of the Quaternary at the onset of North Pole glaciation approximately 3 million years ago and includes portions of the upper Pliocene. Some people do not recognize the Quaternary and consider it an informal term included in the Neogene, as can be seen from the 2003 edition of the International Stratigraphic Chart, published by the International Commission on Stratigraphy. The 1.8-1.6 million years of the Quaternary represents the time which recognizable humans existed. Over this short a time period, the total amount of continental drift was less than 100 km, which is largely irrelevant to paleontology. Nonetheless, the geological record is preserved in greater detail than that for earlier periods, and is most relatable to the maps of today, revealing in the second half of the twentieth century its own series of extraordinary landform changes. The major geographical changes during this time period included emergence of the Strait of Bosphorus and Skagerrak during glacial epochs, which respectively turned the Black Sea and Baltic Sea into fresh water, followed by their flooding by rising sea level; the periodic filling of the English Channel, forming a land bridge between Britain and Europe; the periodic closing of the Bering Strait, forming the land bridge between Asia and North America; and the periodic flash flooding of Scablands of the American Northwest by glacial water. The Great Lakes and other major lakes of Canada, and Hudson's Bay, are also just the results of the last cycle, and are temporary. Following every other ice age within the Quaternary, there was a different pattern of lakes and bays. The climate was one of periodic glaciations with continental glaciers moving as far from the poles as 40 degrees latitude. Few major new animals evolved, again presumably because of the short—in geologic terms—duration of the period. There was a major extinction of large mammals in Northern areas at the end of the Pleistocene Epoch. Many forms such as saber-toothed cats, mammoths, mastodons, glyptodonts, etc., became extinct worldwide. Others, including horses, camels and cheetahs became extinct in North America. Glacial period In 1821, a Swiss engineer, Ignaz Venetz, presented an article in which he suggested the presence of traces of the passage of a glacier at a considerable distance from the Alps. This idea was initially disputed by another Swiss scientist, Louis Agassiz, but when he undertook to disprove it, he ended up affirming his colleague's theory. A year later Agassiz raised the hypothesis of a great glacial period that would have had long-reaching general effects. This idea gained him international fame. In time, thanks to the refinement of geology, it was verified that there were several periods of forward and backward movement of the glaciers and that past temperatures on Earth were very different from today. In particular, the Milankovitch cycles of Milutin Milankovitch are based on the premise that variations in incoming solar radiation are a fundamental factor controlling Earth's climate. During this time, thick glaciers advanced and retreated over much of North America and Europe, parts of South America and Asia, and all of Antarctica. The Great Lakes form and giant mammals flourish in parts of North America and Eurasia not covered in ice. These mammals become extinct when the Ice Age ended about 10,000 years ago. Modern humans evolved about 100,000 years ago. References *The "Quaternary glacial period" section was derived from the article es:Glaciar in the Spanish-language Wikipedia, which was accessed in the version of July 24, 2005. External links * Jan Zalasiewicz, The Guardian, June 30, 2005, "Repackaging time" Category:Quaternary Category:Neogene Category:Cenozoic Category:Physical geography Category:Physical oceanography